1. Field of the Invention
The invention relates to a brake system, and more particularly to a brake system in which cooperation control of a hydraulic braking device and an electric parking brake device is performed.
2. Description of Related Art
A disk brake device and a drum brake device are available as brake devices for vehicles. The brake device of each type is configured such that the hydraulic pressure corresponding to the working liquid delivered from a master cylinder according to the depression amount of a brake pedal operated by a driver is supplied to the wheel cylinder of the disk brake device or drum brake device, thereby generating a braking force (referred to hereinbelow as hydraulic braking force control). A parking brake device that maintains the stopping state of a vehicle when the vehicle is stopped for a comparatively long time or parked is also available. The parking brake device is usually of a manual type in which a braking force is generated when a driver operates a lever, but electric parking brake devices using an electric motor has also been in practical use. In the electric parking brake device, a nut disposed inside a piston is advanced by the electric motor, the piston is pressed, for example, against the brake pad of a disk brake device, and the brake pad is pushed against the disk rotor, thereby generating a braking force.
Meanwhile, vehicles provided with an assist function of performing the braking control automatically according to the state of vehicle surrounds have been also put to practical use. For example, there are systems such that when the host vehicle travels following a vehicle that travels ahead thereof and the vehicle ahead is decelerated or stopped, hydraulic braking force control is executed and the host vehicle is decelerated or stopped without participation of the driver. There are also vehicles such that where the vehicle is automatically stopped and a predetermined period of time elapses thereafter, an electric parking brake device is automatically actuated and controlled to maintain the stopping state of the vehicle (see, for example, Japanese Patent Application Publication Nos. 2007-230277 (JP 2007-230277 A) and 2007-069684 (JP 2007-069684 A).
In this case, the vehicle is decelerated and stopped by performing hydraulic braking force control, for example, of a disk brake device mounted on each wheel (four wheels). After a predetermined time elapses, the electric parking brake devices mounted, for example, on two rear wheels are actuated to maintain the stopping state of the vehicle.
The braking force generated by the brake device is determined by the friction coefficient of the road and a load (ground contact load) applied from the vehicle to the road. Therefore, where a vehicle is decelerated and stopped on a low-μ slope, a stable vehicle stopping posture can be obtained with four wheels, but with two wheels, the vehicle stopping posture changes, and the vehicle occupants can feel uncomfortable. Further, when switching is performed from hydraulic braking force control of four wheels to braking by the two-wheel electric parking brake, a switching shock (vibrations) caused by a change in the number of the braked wheels and a change in the braking method can occur. In addition, when the hydraulic braking force control is automatically executed, a wheel cylinder corresponding to each wheel is in a high-pressure state created by the working liquid. Therefore, even if the driver performs a braking request operation, that is, depresses the brake pedal during the automatic braking force control, the hydraulic liquid cannot flow into the wheel cylinder. Therefore, the stroke of the brake pedal is made difficult, and the driver can feel uncomfortable.